This application is a 371 of PCT/FR99/00632 filed Mar. 18, 1999.
The present invention relates to new products with original fat globule distribution characteristics, the method of obtaining such products and the applications thereof. The preferred field of this invention is the dairy product one.
More particularly, the invention relates to a process for selectively separating according to the size thereof without modifying significantly their membrane integrity, fat globules being present in a food or biological medium. In the following text, the invention has been described more in detail by referring to milk treatment. This one is in fact a particularly interesting case of food liquid adapted to be subjected to said process.
Fats contained in milk produced by mammals (cow, goat, sheep, cow-buffalo, mare, she-ass, women, etc.) are present at more than 95% as spherical globules, visible under optical microscope, with a diameter in the range of 0.1 to 20 xcexcm. As regards cow milk, the average diameter thereof is in the range of 3 and 5 xcexcm and their gaussian dispersion is mainly comprised between 2 and 12 xcexcm (Alais, 1984, Science du Lait, Edition Sepaic). Most of the fat globules (80%) have a diameter lower than 0.1 xcexcm, but only represents a very low proportion in weight of the milk fat material (Keenan et al., 1988, Fundamentals of Dairy Chemistry, Edition Van Nostrand Reinhold). The fat globule size distribution varies slightly according to the species, the feed and the lactation period for dairy cattle (Keenan et al., supra).
Fat globules from cow milk are formed with a membrane having a complex structure comprising numerous protein kinds (20 to 40 according to the authors) with amphiphile properties, like butyrophiline, or enzymatic ones, like xantine oxidase, complex lipids (phospholipids and cerebrosids some of which have also complex glycations, including sialic acid, N-acetylgalactosamine, . . . ) and nucleic acids (Keenan et al., supra) surrounding a droplet of di- and triglycerides, being partially crystallized at room temperature (Alais, supra).
Separation of fat globules from the milk rest, i.e. a so-called phenomenon of skimming, is based upon the volume mass difference (density) existing between globules and the liquid in which they are suspended. Two skimming types are conventionally distinguished: the so-called spontaneous skimming, beforehand quite practiced in manual butter- and cheese-manufacturing, so providing an agglomerated fat globule enriched layer, an operation performed at 5-10xc2x0 C. during 10-16 hr, and the centrifugal skimming where whole milk subjected to a centrifugal rotation of about 4000 to 5000 rpm within a pile of conical disks (Towler, 1986, Modem Dairy Technology, Edition Robinson) is continuously separated into cream and skimmed milk. The cream being obtained according to such a process and used in the worldwide dairy industry is a milk that is strongly enriched with globular fat material (the most current content thereof is 400 g/kg). Depending upon the equipment being used, the fats content of the skimmed milk is equal to or lower than 0.5 g/kg.
There exists a need in products, in particular dairy products, comprising selected fractions of fat globules as well as products containing calibrated fat globules. The dairy industry is looking more and more for new products meeting customers"" needs and amongst them products coming from fat product conversion.
The object of the present invention relates to a new methodology for separating and fractioning through micro-filtration membranes fats contained in a food or biological medium, in particular in the milk produced by dairy females, based not upon volume mass differences between fat globules and skimmed milk like in the prior art, but upon particle size differences of the milk components of mammals.
Thus, the invention does not relate to a total separation of the globular fat material, but to a differential fractioning of said fats according to the globule size so as to implement the surprising properties of the fractions enriched with small and large globules.
The membrane micro-filtration technique is known per se and has been already proposed in the dairy industry in particular. As a pertinent reference, the article of J. L. Maubois titled xe2x80x9cCurrent uses and future perspectives of M. F. Technology in the dairy industryxe2x80x9d in Bulletin of the IDF 320, 1997, pp. 37 to 40, that reviews the skilled man knowledge in that field and mentions numerous bibliographic references.
The total whole milk skimming, i.e. the total globule retention by membrane micro-filtration in tangential flow has been proposed by Piot and al. (La Technique Laitiere nxc2x0 1016, 1987). The membrane being used with an average diameter of 1.8 xcexcm led to a retention of about 98% fats that the authors considered as insufficient with respect to the centrifugal skimming and to improve it they proposed to use micro-filtration membranes with a lower pore diameter.
Such an article does not contain any teaching about the production of fat products containing selected fat globule fractions and highlights that the micro-filtration technique, though a priori useable for skimming and bacterial purification of crude whole milk, raises numerous unsolved technical problems.
U.S. Pat. No. 4,140,806 discloses a process for separating milk through tangential micro-filtration into a first fraction containing almost the total fats comprised in the starting product and into a second fraction being practically fat-free, in filtration conditions that do not allow to produce a substantially uniform transmembrane pressure.
Patent FR 95.02939 (publication 2,731,587) describes a process to remove somatic cells from food or biological media consisting in subjecting the medium to be treated to at least a tangential micro-filtration on a membrane having a cut-point threshold higher than 10 xcexcm, preferably in the range of 10 to 50 xcexcm. A preferred application for this process is milk treatment, the technical problem in such a case being to produce a milk matching the most severe sanitary quality requirements, i.e. the somatic cell rate of which is very low while keeping the common flora useful in milk conversion, for example cheese. Such prior patent only takes problems associated with the presence of milk somatic cells into account and does not contains any teaching upon the production of fat products containing selected fat globule fractions.
The products produced according such a patent are a micro-filtrate containing less than 10 000 somatic cells/ml and containing the essential starting milk fats as well as a retentate representing 4% in volume of the milk being in contact with the membrane and containing the essential part of the somatic cells and a little fat material.
The present invention has for object a process that allows to separate fat globules selectively according their size and then to produce derivates containing calibrated globules. These ones stay in their native condition, that means their membrane has not been modified and stays with the natural properties thereof.
An object of the invention aims at a process for separating selectively according to their size the fat globules present in a food or biological medium consisting in subjecting the medium to be treated to at least a tangential flow micro-filtration on a membrane with a cut-point threshold comprised between 1.8 and 10 xcexcm in substantially uniform transmembrane differential pressure conditions and recovering the retentate and permeate being obtained.
In the case of milk, the resulting products are used for preparing milks and creams falling into the conventional conversion processes or the new processes.
The selective separation is performed on micro-filtration membranes with cut-point thresholds in the range between 1.8 and 10 xcexcm depending upon the selection being looked for. The following illustrative examples demonstrate the interest of selecting membranes with a cut-point threshold higher than 1.8 xcexcm and lower than 10 xcexcm. Although the tangential flow micro-filtration treatment is possible technically outside such a range, it leads, in particular in the case of milk, to less interesting dairy products for their subsequent conversion. Thus, with membranes having pore sizes of 0.8 xcexcm and 1.4 xcexcm, the fats content of the permeate is weak (respectively 5.6 g/kg and 2.4 g/kg). A membrane with pore diameter of 12 xcexcm leads to a separation of very large fat globules.
The invention finds a privileged application to fractionate globular fats contained in the whole milk or in the cream resulting from whole milk skimming.
Depending upon the starting row materials, the invention allows then to obtain new dairy products distinguished from those of prior art through their fat globule repartition.
The invention relates namely to the dairy products below, the two first ones being recovered as a permeate and the last one as a retentate:
(i) dairy product having a total fats content lower than or equal to 400 g/kg and the fat globule repartition of which is such that globules with a diameter lower than 2.0 xcexcm represent at least 10 to 60%, preferably 20 to 40% in weight of total fats and fat globules with a diameter higher than 6 xcexcm represent less than 10 to 30%, preferably 15 to 20%, in weight of total fats;
(ii) dairy product having a total fats content lower than or equal to 30 400 g/kg and the fat globule repartition of which is such that fat globules with a diameter lower than 3.0 xcexcm represent at least 20 to 90%, preferably 30 to 80%, in weight of total fats and fat globules with a diameter higher than 6 xcexcm represent less than 10 to 30%, preferably 15 to 20%, in weight of total fats;
(iii) dairy product having a total fats content higher than of equal to 30 g/kg and the fat globule repartition of which is such that globules with a diameter lower than 5 xcexcm represent less than 90% and preferably less than 80%, in weight of total fats and fat globules with a diameter higher than 6 xcexcm represent more than 10 and preferably more than 20%, in weight of total fats.
In the process of the invention, any mineral, organic or composite type of membrane may be used, with the proviso that the membrane being considered should be suitable for the tangential flow microfiltration installation being used.
The skilled man knows the general characteristics of the tangential flow microfiltration installations and, if necessary, he can refer to the bibliographic references mentioned thereabout in the introduction of the present specification, for example the article from Piot et al. (1987), J. L. Maubois (1997) and French Patent published under no. 2, 731,587. The content of such documents is incorporated by reference into the present specification.
The actual implementation of tangential flow micro-filtration is performed in an installation of known type. The product to be treated is caused by a feed pump to circulate in a tangential flow through channels formed with appropriate membranes or comprising the same. One part of the product, called permeate or micro-filtrate, goes through the membrane and is recovered in an enclosure. The medium, that does not go through the membrane, called retentate, is recycled back to the circulation pump and mixed with the entering product so as to be again subjected to the micro-filtration. In the practice, the installations are thus formed as a ring system. Such rings may be connected in parallel so as to form more powerful installations. An appropriate example for this type of installation is the MFS micro-filtration unit from Alfa Laval.
The process according to the invention may be operated continuously or discontinuously. In that last case, a determined quantity of the medium to be treated is contacted with the membranes of the micro-filtration unit up to the production of a permeate and a retentate with the desired properties. But the tangential flow micro-filtration installations may also be operated continuously and then the medium to be treated is brought in contact with membranes continuously, the operating conditions for the installation being selected so as to be able to recover, also continuously, a permeate and a retentate with the desired properties. The skilled man has available the adjustment means for the micro-filtration units so as to obtain such a result in industrial conditions.
To be subjected to the process according to the invention, it is sufficient if the media to be treated are pumpable so as to brought into contact with the membranes. Such media may then be suspensions or liquids. Thus, they may be as mentioned previously biological media or food media, more preferably milk or a milky product, such as a cream or a lactoserum for example, or a mixture containing one or more milks or one or more milk components. The dairy product being put in contact with the micro-filtration membrane may thus centrifugal skimming or any other means obtain a cream.
Milk may be from any dairy female : cow, goat, sheep, she-ass, cow-buffalo, separately or in mixture. The process according to the invention is particularly convenient for crude whole milk treatment.
The implementation of the process so as to provide membrane selectivity and avoid an obstruction is carried out by observing adequate hydrodynamic conditions, namely:
retentate (product retained by membrane) re-circulated at an average tangential speed of 1 to 10 m/s, preferably 4 to 8 m/s;
permeate (filtrate going through the membrane) co-current re-circulated on the external surface of the filtering membrane substrate. The permeate flow rate is regulated so as to obtain a weak identical transmembrane pressure (between 0.2 and 1.0 bar) along the membrane;
feeding filtration ring at a sufficient pressure to provide a good filtration operation;
temperature: 37 to 55xc2x0 C. Implementation may also be carried out at other temperatures on the condition that the product stays pumpable and is not altered by the thermal treatment.
The permeate co-current re-circulation represents a first embodiment of the process according to the invention allowing to obtain a substantially uniform transmembrane pressure on the whole surface of the filter membrane.
Thus, co-current re-circulation of permeate on the external surface of the filtering membrane substrate allows to obtain an identical charge loss (difference between the inlet pressure and the outlet pressure of fluids circulating on either side of the filtering membrane) in each of the filter compartments and therefore a substantially identical pressure difference on each given point of the filtering membrane, on the whole filtering surface. The co-current re-circulation of permeate is for example disclosed in Swedish Patent nxc2x0 SW 74 16 257 (Sandblom).
The production of a substantially uniform transmembrane pressure on the whole filtering surface constitutes one of the essential characteristics of the preparation process according to the invention so as to obtain the desired separation of globules according to their size.
According to the invention, it has been found that in the absence of a substantially uniform transmembrane pressure the micro-filtration membrane is obstructed rapidly (after about 5 to 20 minutes) and the flat globules do not go through it any more. Such filter obstructing characteristic may be an advantage in processes wherein the production of a practically fat globule free fraction and a fraction containing almost all the fat globules from the starting medium is looked for.
On the contrary, a fat globule passage default would be a serious disadvantage in the implementation according to the invention, since the selective fat globule preparation according to their size would not be possible.
The fat globules should keep their original size, that means they cannot be divided or coalesced through mechanical effects specific to the process, for example caused by pumping (nature and type of the pump, rotation speed, . . . ), circulation (circuit geometry, . . . ), extraction of retentate and permeate (valves, pumps, . . . ) or pressure being applied.
As above-mentioned, the implementation of the process may be discontinuous or continuous, but it is preferred to work in continuous so as to avoid the homogenisation and/or coalescence phenomena for the fat globules. A minimum volume of the circulation rings for retentate and permeate is desirable to limit at a maximum the residence times and thus the period of the mechanical effects on the products in the micro-filtration installation.
When co-current circulation of permeate is made and it is desired to apply a uniform transmembrane pressure, the best results have been obtained with mineral or ceramic membranes such as those made with alpha alumina marketed by Societe des Ceramiques Techniques (France) under the trade mark Membralox or by Socidet Orelis France under the trade mark KERASEP or also membranes of trade mark STERILOX (see article of J. L. Maubois, 1997, supra).
According to another aspect, the filtering membrane is arranged on a macroporous substrate with permeability longitudinal gradient. Such a substrate has such a structure that he possesses a porosity gradient that decreases from one end to the other one of the filtering membrane.
Through such a filter substrate, the hydraulic resistance decreases from one end to the other one of the filtering membrane, which allows to obtain a uniform transmembrane pressure throughout the membrane path.
Such a filter type is advantageously made with ceramics, including the filter substrate described in French Patent Application no. FR 97 04 359.
According to another aspect yet, a dynamic membrane filtration may be also carried out, such as described in French Patent no. 93 06 321 (Publication no. 2 692 441), for example by using organic membranes.
According to such an embodiment of the process of the invention, the filtering membrane and the substrate thereof are arranged on a rotating axis, said device being completed by an arrangement of a rotating disk at a small distance from the micro-filtration membrane.
The rotation of the disk located at a small distance (about 4 mm) of the filtration membrane generates a shearing stress from 50 to 100 times higher than upon a conventional tangential filtration, said shearing stress acting in the three dimensions (radial, tangential and axial). In such a device, the generation of the shearing stress on the wall is decoupled from the transmembrane pressure. Such dynamic membrane filtration processes are also described in U.S. Pat. Nos. 5,037,532, 3,997,447 and 4,956,102.
The process according to the invention involves at least a micro-filtration with a membrane having a cut-point threshold between 1.8 and 10 xcexcm, which means that the process may be performed with a single filtration membrane meeting to such definition or in cascade, i.e. it may be practiced two or more successive micro-filtrations at different cut-point thresholds in the range of 1.8 to 10 xcexcm so as to select a fat globule population.
Also, as known in the micro-filtration technique, the fat globule containing medium may be modified during a micro-filtration by diafiltration, in which case the medium being micro-filtrated or the corresponding retentate is added with another medium or solution, for example skimmed milk, milk ultra-filtration permeate, milk micro-filtration permeate or water.
The invention allows namely to obtain:
(a) permeates with a composition corresponding to production milks for converting:
into yoghurts, for example, a permeate containing 10 g/kg fats;
into consumption milks, pasteurised, UHT, for example a permeate containing 17 g/kg fats;
into cheese, for example, a permeate containing from 17 to 38 g/kg fats;
into slight creams, for example, a permeate containing from 150 to 300 g/kg fats;
(b) retentates, either diafiltered or not, adapted to be used as production milks for the same products as those mentioned for permeates or creams adapted to be used after churning for butter conversion.
The invention will be illustrated with no limitation in the following examples. The measurement methods used for parameters and characteristics of the above-mentioned products are those indicated in the article of M. Piot et al. (1987), supra.